KS>>                myStrings[i] = rnd.Next(1000000).ToString("{0:D6}"); // 6 digits
KS>>                patterns[i] = rnd.Next(2) == 0 ? rnd.Next(1000).ToString() : myStrings[i].Substring(3);
KS>>

U.StartWith: 00:00:00.041
String.StartsWith: 00:00:00.052

public static class MyExtentionMethods
{
    public static bool AsciiStartsWith(this string str, string pattern)
    {
            if (pattern.Length > number.Length) return false;
            bool match = true;
            for (int digitIndex = 0; digitIndex < pattern.Length && match; digitIndex++)
            {
                match = match && number[digitIndex].Equals(pattern[digitIndex]);
            }
            return match;
    }
}